5.4. Emission-Line Profile Variability
If the BLR velocity field is ordered (i.e., V = f(r)), then the emission-line profiles should vary in response to continuum changes: the propagation of excitation inhomogeneities through the BLR should result in irregularities in line profiles that evolve on time scales comparable to the emission-line lags. Peterson et al. 73 have examined profile variations in several Seyfert galaxies that were monitored fairly intensively over several years. They conclude that profile variations are not obviously correlated with either the continuum or the total emission-line flux; profile variations are not reverberation effects. They also find that most H profiles can be modelled fairly well with three Gaussian components that are fixed in location and width and vary only in relative flux, which thus allows multiple-peaked profiles and variable asymmetry. They also conclude that profile irregularities can occur quickly, appearing on time scales as short as days or weeks, but they can then persist for a very long time, months or even years. Sample H profiles for two well-studied Seyferts are shown in Fig. 39.
Figure 39. Emission-line profiles for H in NGC 5548 (left-hand column; International AGN Watch data) and Akn 120 (right-hand column; data from Peterson et al. 69). These are representative profiles obtained over several years of monitoring. The H narrow-line component is particularly noticeable in the spectra of NGC 5548.